In the industrial production of potato starch from potatoes, the so-called potato pulp, the fibrous mass of the potato, is among the by-products obtained. Similar by-products or waste products are known from other agriculturally based productions, e.g., sugar beet slices from the production of sugar from sugar beets, citrus peels and citrus pulp from the manufacture of juice and ethereal oils from citrus fruits, and pomace residues from cider production.
These plant by-products are often considered as waste products to be disposed of in the most appropriate and most inexpensive way. However, it will be understood that there may be quite obvious advantages by further developing such plant by-products into products of more commercial value.
A common feature of these by-products is that they consist essentially (about 80-100%) of soluble and insoluble plant fibres, of which about 60-80% are dietary fibres, including three biopolymers: cellulose, hemicellulose and pectin involved in the structure of all plant cell walls, which can be conceived as a cellulose-hemicellulose-pectin network in which pectin, apart from being a structural element, also constitutes the “cement” imparting rigidity to the plant cells. This complex structure in which pectin is attached to the other cell wall components by covalent bonds, hydrogen bonds, and/or ionic interaction is often termed protopectin. Pectin, per se, can be obtained by controlled, acidic or basic hydrolytic extraction of protopectin.
Pectin is a linear polymer composed of units of a-D-galacturonic acid attached by a-1,4-glycoside bonds to form long chains of polygalacturonic acid. The galacturonic acid units are esterified with methanol to a varying degree. A distinction is thus made between high-ester pectin having a degree of esterification (DE) of greater than 50% and low-ester pectin having a degree of esterification of less than 50%. The degree of esterification is defined as the number of methyl-esterified galacturonic acid units expressed as a percentage of the total galacturonic acid units in the pectin molecule and may thus be a value between 0% and 100%.
In pectin from some types of plant material, e.g. potatoes and sugar beets, a varying part of the galacturonic acid units may, in addition, be acetylated, expressed as the degree of acetylation (DAc), which is defined, analogous to the degree of esterification, as the number of acetylated galacturonic acid units as a percentage of all galacturonic acid units.
Neutral sugars, such as galactose, glucose, rhamnose, arabinose and xylose, may also be part of the pectin polymer as side-chains to or as members in the polygalacturonic acid chain.
Hemicellulose is a heterogeneous group of polysaccharides containing several kinds of hexose and pentose sugars and, in some cases, residues of uronic acid. These polymers are classified according to the type of sugar residues being dominant and are individually referred to as xylanes, arabinogalactans, glucomannans and so on.
It is known in the prior art how to de-esterify pectin and pectin-containing materials with acids or bases.
Thus, U.S. Pat. No. 5,567,462 discloses a method of preparing pecto-cellulosic compositions and pectin from pectin-containing plant raw materials, such as citrus peels, sugar beet pulp, sunflower residues, and pomace residues. The method consists of treating the comminuted plant raw materials with an acid, e.g., phosphoric or nitric acid, or with a base, e.g., sodium hydroxide or sodium carbonate, to give a mixture consisting of a solid phase containing cellulose components and a liquid phase containing dissolved pectin. The mixture is mashed, neutralised and finally dried to form pecto-cellulosic dry matter. The mashed mixture may also be separated into a solid and a liquid phase, which are neutralised individually and dried to give a pectin product and a pecto-cellulosic product.
It is also known in the prior art how to de-esterify pectin or a pectin-containing material with aqueous ammonia in an organic solvent, e.g. isopropanol, in which pectin is non-soluble. This technique is for example disclosed in U.S. Pat. No. 2,480,710.
DE-4,013,765 and DE-4,042,405 disclose the preparation of amidopectin and dietetic fibres and fillers in the treatment of dried plant residues, e.g. sugar beet pulp, with gaseous ammonia and subsequent extraction with water or highly diluted sodium hydroxide. By adding acid to the concentrated extract, the result is amidopectin, and the insoluble residue following the extraction results in dietetic fibres and fillers.
It has now been found that, without dissolving pectin and without using organic solvents, it is possible to carry out the de-esterification of pectin in an entirely aqueous system in the treatment of a pectin-containing plant material, which has been swollen in an aqueous solution containing neutral salts prior to the de-esterification treatment.
Thus, it is a purpose of the present invention to provide a new and simple method of treating pectin-containing plants under de-esterifying conditions, thereby achieving fibre-containing pectin products as well as isolated pectin products of high commercial value and high practical use.